The present invention is directed to heat sinks in general, and more particularly to heat sinks for use in dissipating waste heat generated by electrical or electronic components and assemblies.
High power electrical and electronic components continue to have an increasing demand for higher power dissipation within a relatively confined space. In order to provide for such higher power dissipation requirements while remaining suitably compact, several levels of thermal management are usually required at the device, sub-assembly and component level.
At the component level, various types of heat exchangers and heat sinks have been used that apply natural or forced convection or other cooling methods. A typical heat sink for electrical or electronic components employing pin fins and a foam is depicted in FIG. 1. As shown, the heat sink includes a heat spreader plate 10 to which metal pins 12 are attached. An electronic component is attached to face 14 of spreader plate 10 and a cooling fluid, such as air or water, is passed through foam 16 and across pins 12 to dissipate the heat generated by the electronic component.
In demanding applications, the method of heat exchange is usually forced convection to the cooling fluid. In such systems, heat exchange can be improved by increasing the pin surface area exposed to the cooling fluid. This is accomplished by increasing the number of the pins per unit volume. However, there are limitations to achievable pin densities based upon manufacturing constraints and cooling fluid flow requirements.
Reticulated foams are also known in the art for their ability to conduct heat such as the metal foams disclosed in U.S. Pat. Nos. 3,616,841 and 3,946,039 to Walz, and the ceramic foams disclosed in U.S. Pat. No. 4,808,558 to Park et al. Metal foams have been sold under the trade name DUOCEL available from Energy Research and Generation, Inc., Oakland, Calif.
Until recently, metal and ceramic reticulated foams have not been adapted for use in heat sinks for dissipating waste heat from electronic components. However, these structures, especially when comprised of metal, make excellent heat exchangers because of their conductivity and their extremely high surface area to volume ratio. While earlier porous heat exchangers had up to 100 open cells per square inch, reticulated foam has up to 15,625 open cells per square inch. Reticulated foam is far more porous and has far more surface area per unit volume (1600 square feet/cubic foot) than heat exchangers having other structures. The pressure drop of fluids flowing through reticulated foam is also relatively low so that movement of a cooling fluid through the foam is practical.
Studies by Bastawros have now shown the efficacy of metallic foams in forced convection heat removal for cooling of electronics. See, Bastawros, A.-F., 1998, Effectiveness of Open-Cell Metallic Foams for High Power Electronic Cooling, ASME Conf. Proc. HTD-361-3/PID-3, 211-217, and Bastawros, A.-F., Evans, A. G. and Stone, H. A., 1998, Evaluation of Cellular Metal Heat Transfer Media, Harvard University report MECH 325, Cambridge, Mass. Bastawros demonstrated that the use of metallic foam improved heat removal rate with a moderate increase in the pressure drop. Bastawros"" results were based on thermal and hydraulic measurements (on an open cell aluminum alloy foam having a pore size of 30 pores per inch) used in conjunction with a model based upon a bank of cylinders in cross-flow to understand the effect of various foam morphologies. The model prediction was extrapolated to examine the trade-off between heat removal and pressure drop. The measurements showed that a high performance cellular aluminum heat sink (i.e., aluminum foam) removed 2-3 times the usual heat flux removed by a pin-fin array with only a moderate increase in pressure drop.
Thus what is desired is a heat sink having pins and a foam surrounding the pins that is optimizable for a given application.
One aspect of the present invention is a heat sink for electronic devices comprising a spreader plate having a top surface and having a bottom surface wherein a portion thereof is defined for affixing an electronic device to be cooled thereto. A plurality of columnar pins are spaced apart one from the other in a non-uniform manner and affixed to the top surface of the spreader plate substantially perpendicular thereto. A highly porous heat conducting reticulated foam block substantially fills a space defined by said spaced apart columnar pins.
Another aspect of the present invention is a method of making a heat sink comprising a spreader plate, a plurality of columnar pins and a reticulated foam block for filling the space between the columnar pins, the method comprises the steps of selecting the spreader plate and defining a portion on the bottom surface of the spreader plate for affixing to an electronic device. A plurality of columnar pins are selected and affixed to the top surface of the spreader plate such that the columnar pins are arranged in a non-uniform density on the top surface of the spreader plate. A reticulated foam block is placed to fill the spaces between the columnar pins affixed to the spreader plate and is affixed to the spreader plate and to the plurality of columnar pins.
Yet another aspect of the present invention is a method of cooling electronic components with a heat sink of the type having a spreader plate with a plurality of columnar pins affixed in a non-uniform density to a top surface of the spreader plate and having a reticulated heat conducting foam filling the space between the plurality of columnar pins. The method comprises the steps of affixing the electronic device to a bottom surface of the spreader plate and arranging a first plurality of columnar pins to the top surface of the spreader plate over the electronic device at a greater density than a second plurality of columnar pins over the remainder of the spreader plate. The heat conducting foam is affixed to the spreader plate to fill the space between adjacent ones of the first and second columnar pins. A cooling fluid is passed over the top surface of the spreader plate and through the heat conducting foam.